As shown in FIG. 1, a Passive Optical Network (PON) is mainly formed of Optical Line Terminations (OLTs), an optical splitter, Optical Network Units (ONUs)/Optical Network Terminals (ONTs) and optical fibres for connecting these apparatuses. Each OLT, as a central office end apparatus, is connected to the optical splitter through a backbone optical fibre, and the optical splitter is connected to each ONU through a single branch optical fibre. The PON supports fault protection for the backbone optical fibre and the OLT apparatus, and as shown in FIG. 1, two OLT apparatuses (an OLT1 and an OLT2) which are backups for each other are connected to the optical splitter through independent backbone optical fibres respectively. In a normal case, only an active OLT (the OLT1) communicates with the ONU through the active backbone optical fibre. When a system detects a port fault of the active OLT or a fault of the active backbone optical fibre, a service is switched to the standby OLT (the OLT2) to continue serving the ONU. The OLT1 and the OLT2 in FIG. 1 may be two independent OLT apparatuses, or may be different cards which are backups for each other but are integrated in the same apparatus.
In order to ensure that the OLT processes services of the ONUs synchronously, optical signals sent by all the ONUs are required to reach the OLT at the same time. However, distances from the ONUs to the optical fibre of the OLT are different, so corresponding time is required to be delayed when the ONU sends uplink data to the OLT according to the difference in distance from the ONUs to the OLT, and the delayed time is referred to as Equalization delay (EqD). The ONU may obtain the EqD through the following manner. First, the OLT performs ranging processing on each ONU, obtains Round trip delay (Rtd) from the OLT to each ONU, and calculates EqD of each ONU through the following formula: EqD=Teqd−Rtd, where Teqd is a constant denoting Equalized Rtd, and refers to an Rtd value of the furthest ONU under the OLT; and then sends the calculated EqD to the corresponding ONU. After receiving the EqD, the ONU sets its own EqD.
In the PON fault protection procedure, the inventor finds that the following problems exist in the current PON fault protection. When a service is required to be switched to the standby OLT due to a fault of the backbone optical fibre or a fault of the active OLT in the PON, since lengths of the active backbone optical fibre and the standby backbone optical fibre are different, EqD values of the ONUs after switching are also different, so that the standby OLT is required to perform ranging on all the ONUs again and set new EqD. As the ranging processing of the ONUs is performed serially, the OLT can start the ranging processing of a next ONU only after the ranging processing of a previous ONU is completed. Only after the ranging of all the ONUs is completed, service transmission between the ONU and the OLT can be restored. Therefore, interruption time caused by the fault protection is: service interruption time=Lost Of Signal (LOS) detection time+switching decision execution time+N×each ONU ranging time, where N is the number of ONUs accessing an OLT in a PON. If many ONUs access an OLT in a PON, service interruption time caused by fault protection is long, and service continuity cannot be ensured, which brings poor experience to a user, and reduces satisfaction of the user for services provided by an operator.